Like the book of Elephants and the book of Chariots, this book (Magic) will unlock later in the campaign mode, after you reach 4-star fame.
The appearance of this book on the battlefield is random. You get books by killing heroes, or by claiming towns. I got my book of Magic by claiming a small town, in a small mission (not part of the story ; not a ! mission), soon after I had reached 4-star fame. Before I got the book, I was starting to hear some gossip about magic in the tavern. I may have seen a group of magicians on the battlefield also ... so that may be a prerequisite, but nobody can tell for sure.